As is known, foams consist of bubbles of gas whose walls are thin liquid films. Pure liquids do not foam, and short-lived aqueous foams can be generated by passing a stream of air through a solution containing a surface active agent.
Foams per se are thermodynamically unstable since their collapse is accompanied by a decrease in the total free energy. The addition of a polymer may extend the life-time of such foams by several hours. However, since such foams remain in liquid form, their life-time is generally limited by:
(a) The tendency of water to evaporate from the liquid surface to the air, and PA1 (b) The tendency of liquid to drain from the bubble walls.
These factors lead to a gradual thinning and weakening of the bubble wall until it is no longer themodynamically stable and bursts, i.e. the life-time of the foam is governed by the rate at which water drains or evaporates from the walls of the bubbles.
The addition of preformed polymer is beneficial in increasing the viscosity of the foam solution and thus decreasing drainage and also increasing the water holding capacity of the foam. The properties of such foams, containing various stabilizers such as citrus pectin, starch phosphate and gelatin used in a range of concentrations between 1 to 2% are discussed in an article which appeared in Transactions of the American Society of Agricultural Engineers, Vol. 13, No. 1, pp 1-5 (1970). In addition to the stabilizer, small amounts of binding agents such as CaHPO.sub.4 or glycerol are utilized. As pointed out in the article, mixtures with less than 1% gelatin had little skin-forming ability and were thus not recommended as long-life foams. The foams described in the article were suitable for plant protection, provided that the surrounding temperature was below 5.degree. C; at higher temperatures the foam collapsed.
The yield of foams may be expressed by the expansion factor. For several agricultural applications, e.g. plant protection, the expansion factor should be as high as possible for maximum economy. The expansion factor is defined as the reciprocal of the density. According to the article mentioned above, the foams stabilized with gelatin had expansion factor between 35.3 to 55.